[Ifeffit] Analyzing EXAFS data on Artemis

[Matt Newville]

Hi Alex,

I agree with everything Carlo and Matthew said.

As you say, Li is very light and so the scattering should be weak compared
to Fe.  It will also die out much more quickly with "k" than the Fe (or Se)
scattering will.  So, if you have enough k-range, simply starting your fit
at higher k than normal (perhaps 4 or 5 Ang^-1) or increasing the k-weight
used in the Fourier transform (perhaps to 3 or 4) would de-emphasize the
Se-Li scattering to a level that it was safe(r) to ignore.

FWIW, I would imagine that trying to fit coordination number or sigma^2 for
Se-Li at percent-level concentrations would not work very well. If you did
indeed get values that were clearly telling you that there was definitely
Se-Li scattering contributing, I would wonder if there was something else
going on (say, from another ligand, some multiple scattering, or some other
phase).

The formula for the Einstein temperature is a scale factor times
"coth(theta/(2t)) / (r_mass * theta)"  where t is the temperature, theta
the Einstein temperature, and r_mass the reduced mass of the atoms in the
path.   See

https://github.com/xraypy/xraylarch/blob/726136d0184d9a006546002722b7573f6c675357/larch/xafs/sigma2_models.py#L19

for details.  This will not include S0^2 -- they are conceptually totally
different.

As Carlo said, the sigma^2 in the EXAFS equation does not distinguish
between static and thermal disorder.  But if you have temperature-dependent
data, modeling the sigma2 values as a static offset + a term that depended
on temperature with an Einstein model would be a fine way to go.

Hope that helps!


On Thu, May 27, 2021 at 9:43 AM Alexandros Deltsidis <
adeltsidis at iesl.forth.gr> wrote:

> Dear mailing list,
>
> I am currently analyzing some EXAFS data. I am studying a Lix(C5H5N)FeSe
> system in a temperature grid that extends from 20 K to 300 K and I have 4
> such datasets which correspond to different amount of doping (x). Right
> now, im focusing on fitting the 1st coordination cell, in Artemis for the
> Se K-edge. My starting model is the simple P4/nmm FeSe. So, in my system
> the 1st coordination cell, in the Se K-edge, corresponds to the Se
> (absorber) - Fe (backscatterer) pair. I have 2 questions:
> 1) I realize now, that I have a certain impurity in the high doping range
> on my system, namely Li2Se. I try to include a scattering path from the
> respective Li2Se crystal model in my fits, since a Se (absorber) - Li
> (backscatterer) pair is present in the R-range of my fit in the Forward
> Fourier Transform. My question here is if this makes sense since Li is
> much smaller scatterer compared to Se. In other words, does it make sense
> to look for physical parameters (Li-Se bond length and DW factor
> respectively) of a signal (Se-Li) that is "tucked" in below the main peak
> coming from the "majority" Se-Fe signal in the FFT?
> 2)Also, I'm attempting to extract an Einstein temperature for each of
> those datasets, by utilizing the "eins(T, thetae)" function implemented in
> Artemis. What is the equation that is parametrized here? Does it include
> the s0^2 offset term that accounts for the overall configuration disorder
> in the system? And if that is the case is there same way to separate it
> from the temperature dependent s^2 term?
>
> Thank you in advance,
> Deltsidis Alexandros
> PS:I am attaching a png. file exported from Artemis that is relative to my
> question 1)
>
> Ph.D candidate,
> Institute of Electronic Structure and Laser (IESL),
> Foundation for Research and Technology - Hellas
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-- 
--Matt Newville <newville at cars.uchicago.edu> 630-327-7411
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